Cargo Features

atomic-shim has no features set by default.

[dependencies]
atomic-shim = { version = "0.2.0", features = ["mutex"] }
mutex = crossbeam-utils

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

crossbeam-utils mutex?